A wide variety of systems are now known for combining two video images into a single display. Perhaps best known of these are the analog systems, in which special rasters may be generated, one picture may be superimposed on another, and differently generated images may be combined, or clearly distinguished. The functions usually performed, however, are essentially straightforward in character, such as the generation of split images, the superimposition of an individual against a background, and the display of a character message in a contrasting color against a changing video background. A number of systems are also known and in use for generating pseudocolor displays for digitally generated messages and graphical displays, either under software control or as shown in U.S. Pat. No. 3,911,418.
With the continued development and enhancement of the capabilities of digital graphical display systems, however, such techniques do not enable full realization of the potential of a display, in terms of making the data readily and fully comprehensible to a viewer. For example, a modern data processing system can generate a simulated three-dimensional display of a building, vehicle or other structure, and show the structure as viewed from different angles. However, when the display itself is restricted to lines of one color and one intensity against a contrasting background of uniform color and intensity, the observer must himself visualize relationships in order to interpret the display, because no gradations, shadings or interactive effects can be incorporated. As is evident when one looks at a map in which different colors and intensity variations have been used to depict elevation gradients, vegetation and contrasting geographical features, significant features and relationships are much more readily discerned than in a monochromatic equivalent that seeks to present the same information. As another example, engineers and scientists now employ processors in conjunction with interactive display systems in a wide variety of design and analytical studies, in seeking to analyze the effect of different variables and parameters in a computer model of a system or device. A full color display which is capable of a wide range of subtle presentations of different images will be much more readily comprehensible than the typical trace display on a non-illuminated background, or line display on a color background.
Some workers in the art have made attempts at providing a greater degree of versatility, as evidenced by Eiselen in U.S. Pat. No. 3,976,982 and Gilbert et al. in IEEE Transactions on Computers, November 1976, pp. 1089-1100. In the system of U.S. Pat. No. 3,976,982, an image from a source is combined with an image stored in an accumulating memory under the program control of an external element or control interface, in accordance with instructions which must be externally supplied to the control interface via a signal path. This is essentially a static type of control, inasmuch as the external instruction dictates that a given function will be employed until a subsequent choice is made. Thus although a number of functions may be available, only one can be used at a time for the whole image. The same general characterization can be made of the system of the Gilbert et al. publication, which moreover is limited in the number of functions that can be performed.